The present invention relates to a typing mechanism of a serial printer suitable for use for creating a display for small desk-top calculators or measuring instruments.
Desk-top calculators and measuring instruments having display devices for digitally displaying the calculation or measuring result are known. Recently, there have been proposed desk-top calculators and measuring instruments having a recording function in which the calculation or measuring results are printed on a record paper by means of a serial printer. The serial printer of this kind preferably has a compact construction because only a small space is typically available for the installation of such a printer. It is, however, extremely difficult to reduce the size of the serial printer because it has various complicated mechanisms such as the rotary drive mechanism for the type wheel, the mechanism for effecting column shift of the carriage carrying the type wheel, the hammer driving mechanism for effecting the typing, the paper feed mechanism and so forth.
Current desk-top calculators also display various arithmetic operation functions so that it is required to print function symbols of a number which is often as large as the number of numerals 0 (zero) to 9 (nine) which may be printed. If the numerals and the function symbols are accomodated by a common type wheel, the diameter of the type wheel is increased undesirably to increase the size of the printer as a whole, which makes it difficult to install the printer in the space available in the calculator or the like.
In order to obviate this problem, a printer has been proposed in which separate type wheels are used for the function symbols and numeral symbols, and only the type wheel for the numeral symbols is arranged to move for a shift in column position along a line of type. As will be seen from Japanese Patent Laid-open No. 46930/1977, this proposed printer has separate hammer members associated with respective type wheels and adapted to operate independently of each other, so that a complicated mechanism is required for driving the two hammer members by a single driving source.
Also a serial printer in which the two type wheels are selectively hit by a single hammer member to eliminate the mechanism for the selection between two type wheels has been proposed. This improved serial printer is disclosed in Japanese Patent Laid-open No. 56514/1978.
The serial printers proposed in Japanese Patent Laid-open Nos. 46930/1977 and 56514/1978 are suitable for mounting in desk-top calculators or the like because of their reduced size and weight. These serial printers, however, still involve problems of high cost and large space of the driving source which typically includes a pulse motor for driving the type wheels rotably, a solenoid of a comparatively large size for effecting the column shift of the type wheel and the driving of the hammer member, and another solenoid of comparatively large size for the record paper and release of the carriage to return it to its initial position.
Meanwhile, Japanese Patent Laid-open No. 68325/1979 described a serial printer having two type wheels, in which the selection of type, printing, column shift, paper feed and other necessary operations are made by a single motor. In this serial printer, however, there is a practical limit to which the printing speed may be increased because the motor has to be reversible. In addition, the reversible motor is comparatively expensive and requires a complicated control. Furthermore, a complicated mechanism is required for the selection and holding of two type wheels independently of each other.